When he died in 1946, Alfred Stieglitz, the great photographer and tireless promoter of modern art, left his estate to his wife, the painter Georgia O'Keeffe. His work as a photographer she shrewdly distributed to the large American museums that could be counted on to secure his reputation. But a sizable part of his art collection O'Keeffe deposited in a less predictable place. At the urging of a friend, the Harlem Renaissance writer Carl Van Vechten, she gave 97 works to Fisk University, the historically black school in Nashville. And she threw in a few of her own. One of those...